Flicker photometers, particularly for measuring the contrast of a lens



GCLS, 1i

Filed June ets-Sheet 1 STEPHEN M. MCNEILLE 1 INI/ENTOR v1w E 2; Z

Try a AGT Oct. 5, 1948. s M. MacNEILLE 2,450,761

FLICKER PHOITOMETERS PARTICULARLY FOR MEASURING THE CONTRAST OF A LENSFiled June 9, 1945 2 Sheets-Sheet 2 STEPHEN M. M4CNEILLE INVENTORPatented Uct. 5, 1948 alswel FLICKER PHOTOMETERS, PARTICULARLY:

FOR MEASUBING THE CONTRAST AOF A LENS'v Stephen.. Mr'. MacNeille, OakRidge, Tenn., as-

signor toEastmanKodak Company, Rochester,

N. Y., a corporation ofNew Jersey Application June 9; 1945, Serial No.598,451

(Cl. S814) 11 Claims.

a flicker photometer in which the .relative intensities. ofithe.comparison `fields are a measureoi the contrast of avlens orotheroptical system,

5., able in front of the xed object:

lf lS the prlm'aryi object of the invention' to. In order to test theoptical contrast-of a lens, provide a flicker photometer in Whichthe'corn-Y telescope-orf. the like I have arrangedallicken parison beamstraverse exactly the saine optical photclneter so that the comparisonelds. receive. system'so asto" eliminate'as far as'possibleall lightonly from asinall. portion. of an. image sources lof error;. Priorflicker photometersal- 10. formed bythe lens. In the original. object,this. Ways involvedscme means'for splitting the beams portion is a. spotor other primary area surroundapartlso thatone'traversed a slightlydiierent ed by a background and the relative intensities optical systemfrom" the other. The instrument of the spot'and background are changedto.pro. also has'all of' the advantages of other icker videthe nicher.For examplethe background. photcrneters including the fact that'the testmay 15. may be maintained as -a constant White and .thebe'madevisuallyor'by'means of a photoelct-ric spot may be alternatedbetween black andwhite` cell.- If the lens has perfect contrast none ofthe .back.

In flicker DllOOmetC-ls liest ld and a StandL groundflight will bescattered. intotheimageof ard'COllllDrlrison-eld'ale presentedalternately t0 the black spot and henceno light Will reachthe. the eyeor photoelectric'cell and the-instrument 20 corresponding oomparisoneld.of the photomis adjusted until the-two fields have equalintenseter; Theamount of light scattered into` the. ity aS lldl'aed by lille absenceOffllcker." AC-v spot image is a measure ofthe lens contrast. Al @Ordingto the present'inventon aflickerphot/01'11"-4 second embodiment oi thiscould have a dark eter iS made-11D llClildng SOme manSOr fOTmground witha spot .changing from black to White lng abeam of llgli'if'alternatingintensity inf 25. but this -is less preferable since the contrast wouldaccordance with the factor to berncasured: Two then be measured only bythe decrease in inpola'rizinglters with' their Vibration axesrnututensity Of thewhite spot. A third arrangement alli'l'fatflgllt'nglesWhen' in the bam. aleDOSl'- or series or arraneementshas th'e'centralspot tion'able intthe beam 'alterntngly and'synchronof const-antintensity. black, gray, or.\-.f`r.te'andl ZSdfWllJl'lGVlyDg intensity.Thus the t\vo 3 05A has the background change from black to White.'lters or sections of alter cori-@Spfmd'reSlDC-l Since-the comparisonelds areconned to the tively to Whatever two conditions cause-thealtersporJ images, the differences in the elds will benatingfintensltyofrthe:light beam-l A 'plane DO`- entirely those causedbylight `scattered-into the larizingxlteris.mounted'somewhereinftheflight Spot image when the background changesfrom beam and'isrotatable-to;vary the ratio of 'the a1- 35. black toWhite. I'prefer inthis embodiment to ternating". intensities;` Whenflicker has; been use a black` or a gray, i. e. a medium brightspotelm'ed-S delctedi through an" eyepiece 01'` compared to the intensities`ofthe background.' by'a-photoLelcctric1cell;. the-adjustment' of theItis an object of all of the'embodiments of `he' plane. polarizingltersis-.a measure -of hmvmuchz invention relating tocchi-rast meters toelimfn-.ite thealternaring intensities must be modiedtobe 0, all thesources oiscattered light vor other vara-f equ'alzand hence/:is=an-eindication of their. ratio. tions which might introduce an error inthe read-- ifthezp'lane lpolarizingnlter. Wereabsent.` ing ofthe lenscontrast.

vElzirexarnple;two 'obfects Whose' transmissions According totheinventionas'applied'4 to lens or-reflectancesfare tofbe'compared aremounted contrast meters there is providedan Object lconona'zdiscrrotating through the light beam so that- 45. ssting-ofaspotand abackground of variable the :objects intercept the light beamalternately.- relative intensities; and means for holding the.The.movemenlf` 0f'. the tWO polarizing lters is. lens to be tested sothat it'receives'light from the synchronized-With'. thermovementof theobjects object andfocuses it on a mask 'having' an apex-L alsoto;interceptthe:-light beam'alternately.. ture therein for transmittingonly some or all:

Inmrdertozbe able-.to use -thephotometer for. 50 of the light intheimage of the spot. Theflick'er measuringeither transmission orreilectionl predetecting means is located behind thev'mask to fer tohai/era light source WhiCh'may-be moved t0- receive thelightAtransmitted-throughl the apereither oftwopositions'or to havetwolight-sourcesture. In` a. preferred arrangementincorporatingwhich,may-*beiuseflaliernatvely; One'fOr-.tranS-- the abovedescribedtype of'nicker photometer: illuminating lterszandthelike andtheother. 5o

for-illuminating surfaces from the frr'nt.. When using the instrumentfor comparing reectances,.

one of theobjects may be located permanently and the other mounted to bealternately positione polarizingnlters with their' axes effectively-atThe details and refinements of the various ein! bodiments of theinvention and all of its advantages will be understood from thefollowing description when read in connection with the accompanyingdrawings in which:

Fig. 1 is a schematic perspective view of the' optical system of aphotometer incorpora-ting the invention.

Fig. 2 is a schema-tic showing of a ilicker detecting device which maybe used in place of the eye shownin Fig. 1.

Fig. 3 similarly shows analternative arrangement of the invention.

Fig. 4 is a schematic perspective view, partly cut away, of the opticalsystem of a lens contrast meter according to the invention.

Fig. 5 similarly shows a slightly different lens contrast meteraccording to the invention.

In Fig. 1 light from a lamp lil is collimated by a lens H to form alight beam which eventually is brought to focus by a lens 33 near thefocal plane of an eyepiece 35 to be viewed by the eye 36 of an observer.A transparent disc l2 rotatable on a shaft i3 intercepts the beam oflight. By means of clamps il and i8, two lters i5 and IB whosetransmissions are to be compared are mountedon the transparent disc l2.On the other end of the shaft I3 rotating synchronously with the testobjects is a two-part polarizing iilter. As indicated by the doubleheaded arrows the two parts 2l and 22 have their vibration axeseiectively at right angles, when positioned in the light beam. Actuallyone part has the vibration axis circumferentially oriented and the otherpart has it radially oriented. The shaft i3 is driven by a belt andmotor 25.

Somewhere in the light beam, preferably but not necessarily in theposition shown, there is a plane polarizing filter 3e which isadjustable to control the ratio of the intensities of the two beams. Ifthe lters i5 andi have identical transmission factors, the comparisonbeams will have identical intensities and there will be no flicker, whenthe lter 3Q is oriented at 45 with respect to both the filters 2i and22. Rotation of the filter 3U increases the relative amount of light ofone beam and decreases that of the other. The adjustment of the filter39 is indicated by a pointer 3| and scale 32 which may be calibrated'toread intensities directly. The eye 3B of the observer may be replaced bya photoelectric cell 37 shown in Fig. 2 whose output isA measured by anA. C. amplier and indicator 33. Absence of flicker is indicated by theabsence of any A. C. component in the photocell output.

If the objects i5 and l5 to be compared are opaque and their reectancesare to be measured, the light source l0 and the lens Il may be replacedby a light source 4I and lens 2 positioned to illuminate the objectsfrom the front. Preferably the instrument is made up with both lightsources in position, only one being. turned on at a time.

Fig. 3 diiers from Fig. 1 in that the two section polarizing filter ismade on a cylinder. The sections are shown as En and 5l mounted on aring carried by a wheel 138 which is rotated in a plane at a slightangle to the light beam so that the lter intercepts the light beam atonly one place. The filter is usually orthogonal to the light beam. Thefilter is connected by shaft and bevel gears 4S and l to rotatesynchronously with the test objects l5 and i6.

Fig. 4 illustrates one preferred form of contrast meter whose purpose isto measure the contrast of an image formed by a lens 75. f the lens wereprovided with perfect non-reiiecting coatings on each air-glass surfaceand if no light were scattered by dust or by the mount, the contrast canbe said to be unity since the image formed by the lens should haveexactly the same contrast as the object. However, all factors which tendto redistribute the light or a portion thereof uniformly over the imageplane reduces the contrast of the image by a factor which may be calledthe contrast of the lens.

The lens to be tested is mounted in one side of an integrating sphere B9one-half of which is cut away. in the drawing to illustrate the'interior. Lamps 6| illumina-te the interior of the integrating sphere 60and also send light through an aperture 62in the sphere danietricallyopposite to the test lens 75. With the shutter 65 in the position shown,all light emerging from the sphere through the hole G2 is absorbed in ablack box 63 so as not to re-enter the sphere. Thus the lens l5 views aeld which includes a central black spot and a uniform white backgroundextending to the edges of the field of the lens and beyond. This fieldis focussed by the lens l5 onto a mask I8 having an aperture I9 thereinjust slightly smaller than the image of the hole 62. Thus the aperture79 transmits only light from the image y' of the central spot. If thelens l5 scattered no light whatever and had unit contrast, this imagewould be perfectly black and no light would be transmitted through thehole '19. If however, as is usually the case, there is some scatteringof light by the lens 15, this light received from the background doesreach and pass through the aperture 19 in the image plane.

In order to measure the amount of this scattered light a white shutter65 is arranged to be alternately positionable over the hole 62 in theintegrating sphere 60 thus to occlude the.

bla-ck box 3. When the shutter 65 is in the light beam, it presents tothe lens 'l5 an object which includes a white spot on a whitebackground. The shutter 65 is mounted on a shaft 6G to be rotated by abelt 6l and motor 6B. For synchronous rotation therewith a two-partpolarizing filter is carried by the shaft 66 with the two parts l0 and'H arranged as before' to have their vibration axes at right angles asdetermined in the beams. The relative intensities of the black and whitespot images are determined by rotating a polarizing lter 'l5 along ascale 'Il until the dicker in the beam through the aperture 'i9 is leliminated as indicated by a photocell 8l, amplifier 82 and meter 83.This arrangement has proven eminently satisfactory in practice buttheoretically would be difficult to use as perfection in lenses isapproached since a perfect lens would give zero intensity for one of thefields which could not conveniently be matched by the testinginstrument. However, the perfect lens is not likely to be made for manyyears to come and the many advantages of the arrangement shown in Fig. 4render it preferable for all practical measurements. One advantage isthe fact that light scattered from the front o the lens toward theobject has no appreciable eiect on the nal measurement. Thus themeasurement is con- I veniently vconii'ned directly to' thecharacteristic in' question.

Iii Fig. 5 there is' shown an arrangement which eiimiriates thedisadvantages of Fig. Li-and which utilizes the testingdeviceeiiiciently in the measurement of optical contrast. A lamp 9Gilluminates the interior' of lan integratingsphere .fil through 'anaperture 92. This light may be cut oir'entirelyleavingthe spheredarkened by means of a shutter S4 which is rotatable by bevel'gears4v 95synchronously' withthefpolarizing hlters 'ID and l l, to permitmeasurements to be made as in Fig. 4. The lamp 9i!" alsolluminates agray area lthe term'. grayf being yused to .indicate lthat in generalthe intensity of thellightinthe central spot of-the-object may. haveanyvalue relative to that'ofthe-background? The light reectedg.; from thegray carri.: S5 illuminates` anapertur'e.SI'inthesphere Sl'iwhichaperture is otherwise surrounded by a black tube S9 to ab- 'sorb lightemerging from the sphere through the aperture Si. A lens to be testedlill is mounted in one side of the sphere to View this object and asbefore to focus it on a mask 'I8 having an aperture i9 therein justslightly smaller in diameter than the image of the hole 91. To minimizethe amount of light passing from the sphere through the aperture Sll andbeing re iiected to add to the light being tested, the lens l0! issurrounded by a black tube 102. The black tubes 9S and fG2 arereinements which are not necessary in most practical testing. The lensiQl alternately views a gray spot on a White ground and a gray spot on ablack ground. With a perfect lens the-intensity in the spot image shouldbe unaffected by the change in the intensity of the background. Apractical lens having a contrast less than unity wastes some of thelight from the gray spot when the background is black and throws extralight into the gray spot image when the background is white. Theadvantages of this embodiment are obvious and it is exti'emely useful. Ablack spot (consisting of a hole in a large black interior box) givesthe largest change in intensity with changing background but thecomparison eld being black would require a measuring instrument diierentfrom the type here used; therefore a dark gray spot is preferred.

Having thus described several preferred embodiments of my invention, Iwish to point out that it is not limited to these structures but is ofthe scope oi the appended claims.

I claim:

1. A iiicker photometer comprising means including a light source forforming a beam of light involving a' factor to be measured, means foralternating the intensity of said beam, two pc- .larizirig ltersalternately positionable in the beam with their vibration axes mutuallyat right angles when in the beam, means for alternately andsynchronously with the alternation of the intensity produced by the beamforming means, positioning the two lters in the alternating beam,

a plane polarizing filter mounted for rotary ad ages 0,761.3`

6 alternately positioningthe` twoV objects' in the'v light'beani to beilluminated thereby.

3. A dicker photometer according to claim l in which the beamalternating means includes ai rotatable member intercept-ing the lightfrom the source to provide said alternating intensity, said rotatingmember and said positioning means being directly coupled for synchronousmoveii-tnt.

4. A flicker photometer for comparing the intensities of vlight from twoobjects, comprising ai light source providing a beam of light, meansforl alternately'positioning'the objects .in the light beam tobe'illuminatedthereby and to provideA alternating intensity to the lightbeam, light po. larizing means with two sections alternately positionable in the light beam with the axes olthe-two sectionseffectively at right angles to each other; meansfor interchanging theobjects inthelight beam, means ior interchanging the two l` sections intheV alternating light beam synclu'o-- nously with the objects wherebythe two sections correspond respectively to the two objects, a pianepolarizing filter also in the light beam mounted 'to be rotatable atleast through a small angle,

means for detecting icker in the light beam caused by said interchangingand means for indicating the adjustment of the plane polarizing iilterwhen ril-citer has been eliminated.

5. A flicker photometer according to claim 4 in which the objects andthe polarizing means are both carried to be rotatable about a commonaxis of rotation laterally displaced from the light beam.

6. A flicker photometer according to claim 4 having two positions forthe light source one to give a direct beam for transmission measurementsand the other in front of the objects for front illuminating the samearea of the objects for reflectance measurements.

'7. A iiicker photometer according to claim 4 in which one of theobjects is eiectively fixed to receive light from said source and torefiect it to the flicker detecting means and the other object is hatand is alternately positionable in iront of the first object and to oneside of the light beam.

8. A device for measuring the contrast of image for-ming optical systemscomprising means for providing an object having a primary arca and abackground Whose relative light intensities are variable, means forcyclically Varying said relative intensities, means for holding theoptical system to be tested to receive varying light from the object andto form an image thereof, a mask at the image plane having an aperturesmaller than the image of the primary area for transmitting only thelight reaching a portion of the image of said primary area whereby thelight transmitted by the aperture includes light from said primary areaand scattered light from said background and is varied cyclically bysaid varying means, cyclically variable light modifying means in thelicht beam from the optical system synchronized with said varying meansand including an adjustable member for adjusting the cyclical variationsof the intensity in said beam and for eliminating these variations,means for detecting flicker caused by said differences in intensity andmeans for indicating the adjustment of said intensity adjusting means.

9. A contrast meter according to claim 8 in which said primary area isa, paraxially located spot in the object field.

10. A contrast meter according to claim 8 in which the intensityadjusting means includes a polarizing filter having two sectionsalternately positionable in the light beam with their vibration axesmutually at right angles and a plane polarizing filter also mounted inthe light beam to be rotatable at least through a small angle forcontrolling the relative intensity of the light beams through the twosections of the polarizing means.

11. A device for Imeasiuing the contra-st of image forming opticalsystems comprising means for providing an object having a primary area..

and a background Whose relative light intensities are variable, meansfor cyclically varying said relative intensities, means for holding theoptical system to be tested to receive varying light from the object andto form an image thereof, a. mask at the image plane having an aperturesmaller than the image of the primary area for transmitting only thelight reaching a portion of the image of said primary area whereby thelight transmitted by the aperture includes light from said primaryareaand scattered light from said background and is varied cyclically bysaid varying means, photoelectric means for detecting the variations insaid relative intensities, cyclioally variable and adjustable lightmodifying means synchronized with said varying means for adjusting thevariations in intensity in the light falling on said photoelectric meansand means for indicating the adjustment of said light modi fying means.

STEPHEN M. NMCNEILLE.

REFERENCES CITED UNITED STATES PATENTS Number Name Date Hardy Jan. 8,1935

